The invention relates to a coupling circuit for passing a signal component and removing any DC component included in a composite input signal.
Generally, an opto-electronic reading or detection system includes a sensor or detector followed by a signal amplifier and the decision-making logic system. For such systems, the reading speeds must have a capability over a range of practically zero to some given maximum speed. Such a broad range excludes the use of AC-coupled amplifiers. The use of a DC amplifier is suggested, but these amplifiers are limited by the amount of DC component present in the input signal.
The instant invention removes the DC component which might be present in a signal. In addition, the instant invention is suitable for use in successive stages which are fixed or variable gain amplifying stages in order to achieve a desired amplification of the signal component.
One of the principal objects of the present invention is a coupling circuit for removing a DC component from a composite signal having a predetermined signal range, including a transistor having a substantially constant DC output threshold for a variable drive signal at a predetermined operating region, biasing means connected to the transistor and operable for setting a controllable operating region defining the constant DC output threshold, the operating region corresponding substantially to the DC component of the composite signal, and being at least partially controllable by signals within the predetermined frequency range, and means for applying the composite signal as a driving signal to the transistor to remove from the composite signal the DC component lying below the threshold, and pass the signal component exceeding the threshold, the biasing means having a relatively high time constant with respect to the highest frequency within the frequency range of the composite signal to minimize control of the operating region by the composite signal.
It is advantageous if the coupling circuit includes an input terminal, and the transistor includes a biasing terminal; the biasing means preferably includes a divider circuit including first and second resistors connected between an operating voltage source of a predetermined polarity and ground, and coupled to the biasing terminal of the transistor.
It is further advantageous if the circuit includes rectifying means, such as a diode, connected between the resistors so as to normally pass current therethrough, and coupled to the biasing terminal of the transistor.
The biasing means preferably include a third resistor connected to the input terminal and the rectifying means, and a capacitor coupled to the biasing terminal of the transistor and to ground.
The coupling circuit may be advantageously interconnected between two amplifiers, so as to couple the amplifiers to each other.